The invention relates to the field of the compression of light pulses and, more particularly, to the compression of pulses emitted by laser sources.
The compression/expansion of laser pulses can be applied in many fields.
For example, in particle physics, the possibility of obtaining very high levels of power from a low-frequency laser enables the atomic nuclei to be extracted from their environment of electrons. The laser pulse processing line gives power density values of over 10.sup.18 W/cm2. The technique used consists, broadly speaking, first of all in extending the duration of the laser pulse by about one nanosecond (for example by dispersion in an optic fiber made of non-linear material) and then in modulating the signal obtained and in amplifying the signal thus modulated (for example by pumping with a YAG/Nd.sup.3+ laser). Amplifications of the order of some tens of joules (up to one KJ) may be obtained. The extended and amplified signal is then compressed through diffraction gratings since it is at the maximum of its peak power. Its power is then multiplied by the compression rate. This technique can be used to obtain power values of the order of some petawatts.
Another application relates to high-speed photography at very high rates, designed for example for the analysis of molecular interactions. The problem is that of being able to have extremely brief laser pulses available. When a laser pulse is modulated on a frequency band with a very great width .DELTA.f, its compression gives a pulse with a particularly short duration of 1/.DELTA.f.
It is possible, for example, by the use of an adapted delay line, to obtain pulse times of the order of some femtoseconds that are compatible with such an application. Furthermore, another possible application is aimed at the detection of targets by reception of modulated pulses emitted by laser sources. This type of detection is used, for example, in LIDARS (Light Detection And Ranging devices) or in active imaging systems. At transmission, the laser pulses having a duration T are frequency modulated in order to obtain efficient detection of any target. At reception, the detection system is fitted out with a pulse compression filter that enables the temporary compression of the useful signal received according to the compression rate of T.multidot..DELTA.f. The pulse is then frequency demodulated and the filter then delivers all the frequencies of the pulse. An analysis of these frequencies gives a target detection system.
In these applications, the compression filter constitutes a dispersive delay line whose propagation time, called a delay time, decreases ("downward" filter) or increases ("upward" filter) with the frequency. For example, a "downward" filter, which is the most widely used filter, delays the lower frequencies of the band: the high frequencies get converted into the low frequencies to give a pulse of compressed duration equal to 1/.DELTA.f. Under these ideal conditions, the pulse is shortened to the maximum extent. Unfortunately, with existing dispersive optical delay lines, the performance characteristics of the compression are greatly limited, especially for large bandwidths .DELTA.f of modulation frequency or for long pulse times T. This is the case for the best known delay line, the one designed by Edmond B. Treacy, as described in the IEEE article, "Optical Pulse Compression Wave Diffraction Gratings", Volume QE-5, September 1969, pp. 454 to 458. This delay line is constituted by a pair of parallel gratings etched on two separate plates with parallel faces. The diffraction gratings are used by reflection and the value of the angle of incidence are determined accordingly. An incident wave is then diffracted twice by a double reflection on the gratings.
The limits of existing optical delay lines are due to the fact that, with such delay lines, the variation of the delay line as a function of the frequency is not linear with this frequency.
For large spectral bandwidths .DELTA.f, or for high pulse times T, the linearity of dispersion is very limited. This substantially limits the performance characteristics of the laser pulse compression.